I had a very interesting conversation with a good friend the other day about overcurrent protection of transformers and the protection of primary and secondary conductors in a typical installation. Someone asked him about a specific scenario when he was teaching a class on transformers. The question involved the maximum primary overcurrent protection permitted in Table 450.5(B) in the National Electrical Code and the required secondary protection of conductors based on 240.21(C)(2). A brief review of protection requirements in the NEC and the method of energizing and de-energizing a transformer is necessary.

Let’s first address the primary overcurrent protection of a transformer, located in 450.5(B) and Table 450.5(B) in the 2026 NEC. This overcurrent protection is only for the transformer and may not provide the appropriate protection for the conductors on the transformer’s primary and secondary, unless provided by 240.21(B) and 240.21(C). This concept is critical in understanding this issue. 

Prior to the 1971 NEC, transformer protection on the secondary side could be rated or set at not more than 250% of the rated secondary current of the transformer, and transformers were not required to have an individual overcurrent device in the primary connection. 

Too many transformers were not adequately protected with this application, so starting in 1971, the code included Table 450.5(B), which requires primary and secondary protection for transformers. Table 450.5(B) permits primary protection of transformers with primary currents of 9A or more with primary-­only protection at 125%, but the size of the primary overcurrent protective device (OCPD) can be rounded up to the next size where the current does not correspond to a standard rating of a fuse or circuit breaker. 

Primary and secondary protections

Overcurrent protection on the secondary side for the transformer is not required since the primary is protected at 125%. Table 450.5(B) also permits the transformer primary to be protected up to a maximum of 250% if the secondary of the transformer is protected at 125%. The primary protection cannot exceed 250%; however, the size of the OCPD on the secondary side can be rounded up to the next size where the current does not correspond to a standard rating of a fuse or circuit breaker.

There are two main reasons to use the 250% protection on the transformer’s primary. The first is where more than one transformer is being supplied by a single primary OCPD. This is not done very often. 

The second reason is where the transformer, based on its saturation current (call it the startup current) and a large motor startup current occurs and trips the primary circuit breaker or blows the primary fuse. This often occurs when someone does not know how to turn the transformer on or off or the wrong type of OCPD is used. 

Not to be too basic, but to turn a transformer on, first make sure the entire secondary load is turned off so only the transformer saturation current occurs. Then turn the secondary OCPD on and start turning on each individual secondary branch circuit load. Obviously, there may be other reasons for using 250% protection, but most knowledgeable people use 125% primary protection and 125% secondary protection for the transformer. This provides proper protection of the primary and secondary of the transformer as well as conductors on the primary and secondary.

Let’s look at the requirements in 240.21(C)(2) for protection of transformer secondary conductors, as follows: “A set of conductors feeding a single load, or each set of conductors feeding separate loads, are permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through 240.21(C)(6). Section 240.4(B) is not permitted for transformer secondary conductors.” 

The secondary conductors are protected from short circuits and ground faults by the primary OCPD based on the ratio of the transformer secondary to primary and by overload protection by the secondary OCPD. The last sentence was added in the 2005 NEC to ensure the secondary conductors are fully sized from the secondary connection in the transformer to the secondary OCPD. 

For example, 240.21(C)(2) requires the secondary conductors to be sized not less than the calculated load or the rating of the equipment containing the secondary OCPD. The primary conductors are protected at their ampacity based on the 125% value and the secondary conductors have an ampacity not less than 1/10 of the OCPD size based on the primary-to-secondary voltage ratio. The primary and secondary conductors and the transformer are properly protected from a failure in either the primary or secondary.

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